This invention is related to techniques for programming a structural phase-change material solid state memory device such as those that use a chalcogenide material which can be programmed into different resistivity states to store data.
Solid state memory devices that use a structural phase-change material as the data storage mechanism (referred to here simply as xe2x80x98phase-change memoriesxe2x80x99) offer significant advantages in both cost and performance over conventional charge storage based memories. The phase-change memory is made of an array of constituent cells where each cell has some structural phase-change material to store the cell""s data. This material may be, for instance, a chalcogenide alloy that exhibits a reversible structural phase change from amorphous to crystalline. A small volume of the chalcogenide alloy is integrated into a circuit that allows the cell to act as a fast switching programmable resistor. This programmable resistor can exhibit greater than 40 times dynamic range of resistivity between a relatively crystalline phase (low resistivity) and a relatively amorphous phase (high resistivity). The data stored in the cell is read by measuring the cell""s resistance. The chalcogenide alloy cell is also non-volatile.
A conventional technique for programming a phase-change memory cell is to apply a rectangular pulse of current (having a constant magnitude throughout the pulse) to the cell, at a voltage greater than a switching threshold for the phase-change material, which leaves the cell in the reset state (the material is relatively amorphous and has high resistivity). To change state, this may be followed by the application of a subsequent rectangular lower current pulse, also at a voltage greater than the switching threshold, which programs the cell to a set state (the material is relatively crystalline and has low resistivity). The reset pulse has a higher magnitude of current than the set pulse so that the temperature of the phase change material is raised to Tm, the amorphizing temperature, before the material is rapidly cooled down or quenched by the very sharp decrease in current at the trailing edge of the reset pulse, thereby leaving the material in the amorphous phase. To change into the crystalline phase, the material can be heated back up to an optimum temperature Topt, which is lower than Tm, using a rectangular current pulse of smaller magnitude, and then rapidly cooled down again, this time leaving the material in the crystalline (low resistance) phase.